The present invention relates to a communication system between a road and a vehicle which enables mobile communication between a road and a mobile station by arranging a plurality of road antennas along the road to form a cell on the road. In particular, the present invention relates to a communication system between a road and a vehicle, utilizing an Orthogonal Frequency Division Multiplex (hereinafter, referred to as OFDM) method as a data modulation method.
There is an increasing demand for communications between road controllers and vehicles. On a superhighway, in particular, to operate a vehicle without any burden on the driver and to prevent accidents both for the controller and the driver, a frequently interchange of information is necessary between the road and the vehicle. One type of such a system has been developed, wherein a self-operating system allows a vehicle to run with close communication between the road and the vehicle, which are equipped with various sensors and a camera (see, for example, Japanese Unexamined Patent Publication No. 241495 of Sep. 17, 1996).
For the construction of a driving support system (hereinafter, referred to as xe2x80x9ccommunication system between a road and a vehiclexe2x80x9d) which makes use of the communication with the vehicle for future extension into a self-operating system, it is necessary to provide a communication area (cell) on the road.
To provide such a cell, we may consider laying a leakage coaxial cable along the road. However, has the drawback of this method is that large-scale construction is needed for laying such a cable. In addition, since it is required to locate the leakage coaxial cable at a relatively low position on the ground, the space for which a radio wave propagates in a direction across a traffic lane is disadvantageously short.
On the other hand, if the communication is performed with a plurality of road antennas being arranged on the road at predetermined intervals, a single road antenna can cover a relatively large cell. In this case, each of the road antennas is connected to a central base station of the road controller via an optical fiber and the like.
In the case where the road antennas are provided, when a large-size vehicle comes proximate to a small-size vehicle, it obstructs the view of the driver of the small-size vehicle preventing visual detection of the road antenna from inside the vehicle. In particular, a microwave or a millimeter wave of a high frequency having a small angle of diffraction is blocked. Accordingly, the communication between the vehicle and the road is interrupted, thereby preventing continued communications. Therefore, in order to enable continuous communication between the road and the vehicle, multi-station communication has been proposed. According to this multi-station communication, a plurality of road antennas having an inherent directivity are provided along the road, and radio waves of the same frequency and the same content are emitted from the respective road antennas toward the same cell.
A multi-station communication system is advantageous because such a system has a plurality of propagation paths for radio waves to be emitted and therefore the radio wave avoids being blocked so as to continuously perform smooth communication between a mobile station and a road communication station even when a vehicle runs proximate to a large-size vehicle such as a truck.
In the case of multi-station communication, however, since a plurality of radio waves are emitted by a similar transmission power into the same cell, which causes fading due to multipath, resulting in intense intercarrier interference or intersymbol interference. Consequently, it is essential to remove the effects of such interference for construction of the system.
Generally, a mobile object communication system using a single carrier is likely to be subjected to the effects of intersymbol interference by a multipath delayed wave.
Accordingly, the use of an OFDM modulation method that allows transmission of a plurality of subcarriers obtained by dividing a carrier has been proposed. The OFDM modulation method is advantageous in that the effects of a delayed wave can be eliminated by setting guard time.
Since a carrier is transmitted after being divided into a plurality of subcarriers in the OFDM modulation method, the space between the frequencies of subcarriers is relatively small.
In a mobile communication method, however, when a vehicle is operating, the Doppler effect occurs with the movement of the vehicle. Therefore, in the OFDM modulation method, interference between subcarriers occurs, which leads to the degradation of communication quality.
Accordingly, it is necessary to set the space between the frequencies of subcarriers to be a sufficiently great value (30 to 100 times or more) with respect to the maximum Doppler frequency defined by the assumed running speed of a vehicle.
However, the occupied bandwidth of frequency assigned according to the Radio Law is fixed in a radio wave, and the number of subcarriers is limited when a space between the frequencies of subcarriers is enlarged, thereby adversely reducing the communication capacity.
Particularly, in the case where a large number of vehicles enter a cell, there arises the problem of a reduction of the transmission rate assigned to each vehicle, which prevents high quality data communication.
The inventor of the present invention assumes that the Doppler shift is small when a large number of vehicles enter the cell because there is traffic congestion or a similar condition. Therefore, the inventor also detects that the relationship between the number of subcarriers, if present, and the average running speed of vehicles in the cell can be established.
In the communication system between a road and a vehicle performing communication between a road communication station and a vehicle-mounted mobile station in the cell, utilizing an OFDM modulation method, the object of the present invention is to achieve a communication system between a road and a vehicle, a road communication station and a vehicle-mounted mobile station, in which the interference between carriers does not occur even when a vehicle runs at high speed and the amount of data transmission is ensured even when the vehicle runs at low speed, thereby preventing the degradation of communication quality.
(1) A communication system between a road and a vehicle recited in claim 1 in order to achieve the above object detects the velocity of a vehicle in a cell and enlarges the space between the frequencies of subcarriers of OFDM as the detected velocity of the vehicle becomes higher. Therefore, interference between subcarriers can be avoided by enlarging the space between the frequencies of subcarriers as the Doppler shift increases with the increase of the velocity of the vehicle in the cell in the present invention.
The phrase xe2x80x9cenlarging the space between the frequencies of subcarriersxe2x80x9d means that the number of subcarriers is reduced (thinned out) in the occupied bandwidth of frequency on the assumption that the occupied bandwidth of the frequency assigned according to the Radio Law is fixed in the radio wave.
Accordingly, although data transmission rate inevitably drops, it is assumed from the fact that the velocity of the vehicle in the cell is high and that a small number of vehicles are present in the cell. Since it is also assumed that the total data transmission capacity of all vehicles is small as well, it does not cause any problem.
If the velocity of a vehicle in the cell is low, the space between the frequencies of subcarriers of OFDM is left as it is without being enlarged.
Since the Doppler shift is small if the velocity of the vehicle in the cell is small, it is unlikely that interference between subcarriers will occur even when the space between the frequencies of subcarriers is not enlarged. Moreover, in the case where the velocity of the vehicle in the cell is low, the occurrence of traffic congestion or a similar condition is presumed, the number of vehicles present in the cell is large, and the total data transmission capacity of all vehicles is correspondingly large. Accordingly, since it is preferable that the data transmission rate be high, a small space between the frequencies of subcarriers is advantageous at this point.